Compositions and methods for manufacturing a cathode for a secondary battery

ABSTRACT

Disclosed are compositions and methods for producing a cathode for a secondary battery, where a fluorophosphate of the formula Li x Na 2-x MnPO 4 F is used as an electrode material. Li x Na 2-x MnPO 4 F is prepared by partially substituting a sodium site with lithium through a chemical method. Li x Na 2-x MnPO 4 F prepared according to the invention provides a cathode material for a lithium battery that has improved electrochemical activity.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims under 35 U.S.C. §119(a) the benefit of KoreanPatent Application No. 10-2011-0097846 filed on Sep. 27, 2011, theentire contents of which are incorporated herein by reference.

BACKGROUND

(a) Technical Field

The present invention relates to compositions and methods formanufacturing a cathode for a secondary battery. More particularly, thepresent invention relates to compositions and methods for manufacturinga cathode for a secondary battery, where the cathode includes manganesefluorophosphate with lithium or sodium.

(b) Background Art

As the use of portable small-sized electronic devices has becomewidespread, there has been an active interest in developing new types ofsecondary batteries such as, for example, nickel metal hydrogen orlithium secondary batteries. For example, a lithium secondary batteryuses carbon (such as, e.g., graphite) as an anode composition,lithium-containing oxide as a cathode composition, and a non-aqueoussolvent as an electrolyte. Lithium is a metal that has a very hightendency to undergo ionization; consequently, lithium can achieve a highvoltage. Thus, lithium is often used in the development of batterieshaving high energy density.

In a lithium battery, a cathode composition typically includes a lithiumtransition metal oxide containing lithium in which 90% or greater of thelithium transition metal oxide includes layered lithium transition metaloxides (such as, e.g., cobalt-based, nickel-based,cobalt/nickel/manganese ternary-based, and the like). However, when suchlayered lithium transition metal oxides are used as a cathodecomposition, lattice oxygen within the layered lithium transition metaloxides may become deintercalated and participate in an undesiredreaction under non-ideal conditions (such as, e.g., overcharge and hightemperature), thereby causing the battery to catch fire or explode.

In order to overcome the disadvantages of such layered lithiumtransition metal oxides, researchers have considered cathodecompositions having a spinel or olivine structure. In particular, it hasbeen suggested that a cathode composition including a spinel-basedlithium manganese oxide having a three dimensional lithium movementpath, or a polyanion-based lithium metal phosphate having an olivinestructure, instead of a layered lithium transition metal oxide, mayprevent problems in lithium secondary batteries that arise fromdecreased stability in layered lithium transition metal oxides as aresult of cathode deterioration.

The use of the spinel-based lithium manganese oxide as a cathodematerial has been limited because repeated cycles of battery chargingand discharging results in lithium elution. Moreover, spinel-basedlithium manganese oxide containing compositions display structuralinstability as a result of the Jahn-Teller distortion effect.

The use of olivine-based lithium metal phosphates, such as iron(Fe)-based phosphate and manganese (Mn)-based phosphate, as a cathodematerial has also been limited because these compounds have lowelectrical conductivity. However, through the use of nano-sizedparticles and carbon coating, the problem of low electrical conductivityhas been improved, and thus the use of olivine-based lithium metalphosphates as a cathode composition has become possible.

For example, it has been recently reported that fluorophosphates may beuseful as a cathode material. The fluorophosphate has the followingformula: A₂MPO₄F, where A represents Li or Na, and M represents atransition metal such as Mn, Fe, Co, Ni, V, or a mixture thereof.Theoretically, the fluorophosphate of formula A₂MPO₄F is expected tohave a capacity about twice as high as a conventional lithium metalphosphate since it has two Na atoms. For example, in the case where afluorophosphate having the formula Na₂MPO₄F (M=Mn, Fe, Co, Ni, V or amixture thereof) is used as a cathode material for a lithium secondarybattery, sodium is deintercalated during the initial charging step andlithium is intercalated during the initial discharging step. In thefollowing cycles of battery charging and discharging, alternatingintercalation and deintercalation of lithium occurs during the chargingand discharging process. Similarly, in the case where Na₂MPO₄F (M=Mn,Fe, Co, Ni, V or a mixture thereof) is used as a cathode material for asodium battery, the intercalation and deintercalation of sodium iscarried out during charging and discharging.

U.S. Pat. No. 6,872,492 discloses an example of using a fluorophosphateincluding sodium, such as NaVPO₄F, Na₂FePO₄F, or (Na,Li)₂FePO₄F, as acathode material for a sodium based battery. However, the example islimited to a sodium based battery, and has not been attempted for alithium battery.

As another example of the conventional art, sodium iron fluorophosphate(Na₂FePO₄F) has been used as a cathode material for a lithium secondarybattery, and the structure of Na₂FePO₄F and its electrochemicalcharacteristics have been disclosed. However, iron-based Na₂FePO₄Fsuffers from a major disadvantage as a cathode material because it has alow charge/discharge potential (about 3.5 V), which is similar to aniron-based olivine material. Attempts to overcome this disadvantage ofNa₂FePO₄F have been made by using manganese-based Na₂MnPO₄F, which has ahigher potential (4V) compared to iron-based Na₂FePO₄F. Unfortunately,Na₂MnPO₄F also suffers from a major disadvantage as a cathode materialbecause of electrochemical inactivity due to the low electricalconductivity of a polyanion-based material.

The above information disclosed in this Background section is only forenhancement of understanding of the background of the invention.

SUMMARY OF THE DISCLOSURE

The present invention provides compositions and methods of manufacturinga cathode for a lithium secondary battery, in which nano-sized particlesreduce the diffusion length of lithium, and the deintercalation ofsodium and the intercalation of lithium are carried out through achemical method. Consequently, in the improved cathode of the invention,the diffusion path of lithium can be previously established, therebyimproving the electrochemical properties of the cathode of the lithiumsecondary battery.

In one aspect, the present invention provides a composition for asecondary battery cathode that includes a compound represented by theformula Li_(x)Na_(2-x)MnPO₄F, which is prepared by chemicalintercalation of lithium into Na₂MnPO₄F.

In another aspect, the present invention provides a method for preparinga cathode for a secondary battery, the method including: (i)synthesizing Na₂MnPO₄F with a controlled particle size; and (ii)carrying out lithium intercalation and sodium deintercalation by an ionexchange method.

According to one aspect of the present invention, it is possible to usemanganese fluorophosphate including lithium as a cathode material with ahigh electrochemical potential by chemically intercalating lithium intoNa₂MnPO₄F with a controlled particle size through an ion exchangemethod. Advantageously, this method establishes a lithium diffusiblesite or pathway within the cathode material, thereby making it possibleto obtain a high charge/discharge property, compared to that of acathode produced from the same size of a Na₂MnPO₄F cathode material notincluding lithium. According to the invention, when the above describedcathode material is applied to the cathode of a secondary battery, it ispossible to achieve a maximum capacity of at least 4 times higher thanthat of the same primary particle size of Na₂MnPO₄F.

Other aspects and exemplary embodiments of the invention are discussedinfra.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features of the present invention will now bedescribed in detail with reference to certain exemplary embodimentsthereof illustrated in the accompanying drawings which are givenhereinbelow by way of illustration only, and thus are not limitative ofthe present invention, and wherein:

FIG. 1 shows a crystal structure of Li_(x)Na_(2-x)MnPO₄F, in whichoctahedral MnO₄F₂ and tetrahedral PO₄ form a framework, and there existsa channel through which lithium and sodium can be intercalated anddeintercalated;

FIG. 2 shows electron microscopic images of a cathode material preparedby Example 3 of the present invention, in which FIGS. 2 a and 2 b showthe cathode material before and after ion exchange, respectively;

FIG. 3 shows charge/discharge curve graphs of a cathode materialprepared according to the methods of Example 1 of the present invention,at room temperature;

FIG. 4 shows charge/discharge curve graphs of a cathode materialprepared according to the methods of Example 1 of the present invention,at room temperature; and

FIG. 5 shows discharge curve graphs of cathode materials preparedaccording to the methods of Examples 1 to 3, in which a change in adischarge curve according to the amount of intercalated lithium isshown.

It should be understood that the appended drawings are not necessarilyto scale, presenting a somewhat simplified representation of variouspreferred features illustrative of the basic principles of theinvention. The specific design features of the present invention asdisclosed herein, including, for example, specific dimensions,orientations, locations, and shapes will be determined in part by theparticular intended application and use environment.

DETAILED DESCRIPTION

Hereinafter reference will now be made in detail to various embodimentsof the present invention, examples of which are illustrated in theaccompanying drawings and described below. While the invention will bedescribed in conjunction with exemplary embodiments, it will beunderstood that the present description is not intended to limit theinvention to those exemplary embodiments. On the contrary, the inventionis intended to cover not only the exemplary embodiments, but alsovarious alternatives, modifications, equivalents and other embodiments,which may be included within the spirit and scope of the invention asdefined by the appended claims.

Unless specifically stated or obvious from context, as used herein, theterm “about” is understood as within a range of normal tolerance in theart, for example within 2 standard deviations of the mean. “About” canbe understood as within 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%,0.1%, 0.05%, or 0.01% of the stated value. Unless otherwise clear fromthe context, all numerical values provided herein are modified by theterm “about.”

Ranges provided herein are understood to be shorthand for all of thevalues within the range. For example, a range of 1 to 50 is understoodto include any number, combination of numbers, or sub-range from thegroup consisting of 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15,16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33,34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, or 50,as well as all intervening decimal values between the aforementionedintegers such as, for example, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8,and 1.9.

The present invention provides a cathode material for a lithiumsecondary battery, which includes a manganese-based fluorophosphatecompound represented by the following Formula:Li_(x)Na_(2-x)MnPO₄F

The cathode material for a secondary battery, which includes the Formulaabove, has a primary particle size of about 300 nm or less, is coatedwith carbon for improvement of conductivity, and shows a potentialplateau in discharging of about 3.7 V to about 4.0V.

The present invention also provides a method for producing a cathodematerial for a lithium secondary battery, the method including:

(i) uniformly mixing sodium (Na) oxide or a precursor thereof, manganese(Mn) oxide or a precursor thereof, phosphate (P) or a precursor thereof,and fluoride (F) or a precursor thereof through ball milling, andcarrying out pretreatment on the obtained mixture, followed by firing tosynthesize Na₂MnPO₄F; and

(ii) intercalating lithium into the cathode material synthesized in step(i) through an ion exchange method to synthesize Li_(x)Na_(2-x)MnPO₄F.

According to a preferred embodiment of the present invention, in step(i) the mixture is uniformly mixed for about 6 hours through ballmilling, and then subjected to pretreatment under an air atmosphere atabout 300° C. for about 2 hours.

According to a preferred embodiment of the present invention, step (ii)includes the step of intercalating lithium ions into the cathodematerial obtained from step (i) through lithium intercalation/sodiumdeintercalation by an ion exchange method.

According to a preferred embodiment of the present invention, step (ii)includes the step of chemically deintercalating sodium from the cathodematerial synthesized from the step (i), and chemically intercalatinglithium into the cathode material.

According to a preferred embodiment of the present invention, thecathode material obtained from step (ii) is uniformly mixed with acarbon conductive material at a ratio of about 60:40 to about 90:10,followed by ball milling. It is contemplated within the scope of theinvention that the aforementioned range includes all sub-ranges withinthe specified range. For example, the ratio of cathode material tocarbon conductive material may range from about 60:40 to about 61:39,62:38, 63:37, 64:36, 65:35, 66:34, 67:33, 68:32, 69:31, 70:30, 71:29,72:28, 73:27, 74:26, 75:25, 76:24, 77:23, 78:22, 79:21, 80:20, 81:19,82:18, 83:17, 84:16, 85:15, 86:14, 87:13, 88:12, 89:11, or 90:10.Similarly, the ratio of cathode material to carbon conductive materialmay range from about 90:10 to about 89:11, 88:12, 87:13, 86:14, 85:15,84:16, 83:17, 82:18, 81:19, 80:20, 79:21, 78:22, 77:23, 76:24, 75:25,74:26, 73:27, 72:28, 71:29, 70:30, 69:31, 68:32, 67:33, 66:34, 65:35,64:36, 63:37, 62:38, 61:39, or 60:40. It is further contemplated withinthe scope of the invention that the ratio of cathode material to carbonconductive material may include all intervening ratios, for example,about 60:40, about 61:39, about 62:38, about 63:37, about 64:36, about65:35, about 66:34, about 67:33, about 68:32, about 69:31, about 70:30,about 71:29, about 72:28, about 73:27, about 74:26, about 75:25, about76:24, about 77:23, about 78:22, about 79:21, about 80:20, about 81:19,about 82:18, about 83:17, about 84:16, about 85:15, about 86:14, about87:13, about 88:12, about 89:11, and about 90:10, as well as allintervening decimal values. Then, the carbon conductive material isuniformly coated on a cathode surface to improve the electricconductivity.

The precursor of the sodium oxide may be selected from sodium phosphate,sodium carbonate, sodium hydroxide, sodium acetate, sodium sulfate,sodium sulfite, sodium fluoride, sodium chloride, sodium bromide, andany mixture thereof.

According to the present invention, the precursor of the manganese oxidemay be selected from manganese metal, manganese oxide, manganeseoxalate, manganese acetate, manganese nitrate, and any mixture thereof.The precursor of phosphate may be selected from ammonium phosphate,sodium phosphate, potassium phosphate, and any mixture thereof.

Furthermore, LiBr or LiI may be used to cause ion exchange betweenlithium and sodium during the intercalation of lithium by the ionexchange method. The carbon conductive material may be citric acid,sucrose, Super-P, acetylene black, Ketchen Black, carbon or anycombination of the foregoing.

Hereinafter, the present invention will be described in more detail withreference to the accompanying drawings.

The present invention provides a cathode material for a secondarybattery, which includes a compound represented by the Formula:Li_(x)Na_(2-x)MnPO₄F, where 0<x<2.

In an exemplary embodiment, the cathode material includes both lithiumand sodium, shows a potential discharge plateau at about 3.7 V to about4.0V, and is coated with carbon for conductivity improvement.

Hereinafter, the method for producing a cathode material for a secondarybattery, according to the present invention will be described. Thespecific production method will be more easily understood through thefollowing Examples.

For example, the cathode material Na₂MnPO₄F for a secondary battery isprepared by uniformly mixing sodium oxide or a precursor thereof,manganese oxide or a precursor thereof, phosphate or a precursorthereof, and fluoride or a precursor thereof through ball milling,carrying out pretreatment on the mixture, and carrying out heattreatment by firing the mixture obtained from the pretreatment.According to the invention, the prepared Na₂MnPO₄F has a particle sizeof about 1 μm or less, and an average particle size of about 300 nm.Na₂MnPO₄F prepared according to the invention is introduced into anacetonitrile solution including, for example, LiBr dissolved therein.Then, Argon gas is flowed into the solution while the temperature israised so that ion exchange between lithium and sodium can be carriedout. By washing and drying the resultant product of the ion exchange, acathode material, manganese fluorophosphate Li_(x)Na_(2-x)MnPO₄F, isobtained.

In order to increase electrical conductivity, the obtained cathodematerial, Li_(x)Na_(2-x)MnPO₄F, was subjected to carbon coating.

The precursor of the sodium oxide may be any suitable sodium containingcompound including, but not particularly limited to, sodium carbonate,sodium hydroxide, sodium acetate, sodium sulfate, sodium sulfite, sodiumfluoride, sodium chloride, sodium bromide, and any mixture thereof.

The precursor of the manganese oxide may be any suitable manganesecontaining compound including, but not particularly limited to,manganese metal, manganese oxide, manganese oxalate, manganese acetate,manganese nitrate, and any mixture thereof.

The precursor of phosphate may be any suitable phosphate containingcompound including, but not particularly limited to, lithium phosphate,sodium phosphate, potassium phosphate and any mixture thereof.

The precursor of fluorine may be any suitable fluorine containingcompound including, but not particularly limited to, metal fluoride,fluoride, and a mixture thereof.

The lithium source used for the ion exchange may be any suitable lithiumcontaining compound including, but not particularly limited to, LiBr,LiI, or any lithium compound mixture suitable for causing ion exchange.

The solvent used for the ion exchange may be any solvent suitable forincluding, but not limited to, acetonitrile. The carbon conductivematerial may be, but is not particularly limited to, citric acid,sucrose, super-P, acetylene black, Ketchen Black, or any suitable carbonmaterial.

The cathode material of the exemplary embodiment of the presentinvention prepared as described above may be used for manufacturing alithium secondary battery. Herein, the manufacturing method is the sameas a conventional lithium secondary battery manufacturing method exceptfor the application of the cathode material. Hereinafter, theconfiguration and the manufacturing method of the secondary battery willbe briefly described.

First, in a manufacturing process for a cathode plate using theinventive cathode material, the cathode material is added with one, two,or more kinds of conventionally used additives, such as, for example, aconductive material, a binding agent, a filler, a dispersing agent, anion conductive material, and a pressure enhancer, as required, and themixture is formed into a slurry or paste with an appropriate solvent(such as, e.g., an organic solvent). Then, the obtained slurry or pasteis applied to an electrode supporting substrate by an appropriatetechnique such as, for example, the “doctor blade” method, etc., andthen dried. Then, through pressing by rolling a roll, a final cathodeplate is manufactured.

According to the invention, examples of the conductive material includegraphite, carbon black, acetylene black, Ketchen Black, carbon fiber,metal powder, and the like. The binding agent may include, but is notlimited to, PVdF, polyethylene, and the like. The electrode supportingsubstrate (collector) may include, but is not limited to, a foil or asheet made of copper, nickel, stainless steel, aluminum, carbon fiber,or the like.

By using the cathode plate prepared as described above, a lithiumsecondary battery is manufactured. The lithium secondary battery may bemanufactured into a variety of different shapes including, but notlimited to, a coin shape, a button shape, a sheet shape, a cylindricalshape, or a square shape. Also, an anode, an electrolyte, and aseparator for the lithium secondary battery are the same as those usedin a conventional lithium secondary battery.

According to the exemplary embodiment of the present invention, theanode material may be a graphite-based material that does not includelithium. Additionally, the anode material may also include one, two, ormore kinds of transition metal composite oxides including lithium. Theanode material may also include, silicon, tin, etc.

The electrolyte may be, but is not limited to, a non-aqueous electrolyteincluding lithium salt dissolved in an organic solvent, an inorganicsolid electrolyte, or a composite of an inorganic solid electrolyte. Thesolvent for the non-aqueous electrolyte may be, but is not limited to,one, two, or more solvents selected from the group including esters(such as, e.g., ethylene carbonate, propylene carbonate, dimethylcarbonate, diethyl carbonate, methyl ethyl carbonate), lactones (suchas, e.g., butyl lactone), ethers (such as, e.g., 1,2-dimethoxy ethane,ethoxy methoxy ethane), and nitriles (such as, e.g., acetonitrile).Examples of lithium salt of the non-aqueous electrolyte may include, butis not limited to, LiAsF₆, LiBF₄, LiPF₆, or the like.

Also, as the separator, a porous film prepared from a polyolefin suchas, for example, PP and/or PE, or a porous material such as non-wovenfabric may be used.

EXAMPLES

Hereinafter, the following examples are provided to further illustratethe invention, but they should not be considered as the limit of theinvention. The following examples illustrate the invention and are notintended to limit the same.

Example 1

Sodium carbonate (Na₂CO₃), manganese oxalate.hydrate (MnC₂O₄.2H₂O),sodium fluoride (NaF), sodium hydrogen carbonate (NaHCO₃), and ammoniumphosphate (NH₄H₂PO₄) were introduced in predetermined amounts withrespect to the total amount of 10 g, and ball milled for 6 hours touniformly mix the materials.

The resulting mixture was subjected to pretreatment at 300° C. for 2hours under an air atmosphere, and fired at 500° C. for 6 hours under anargon gas atmosphere. Then, the resulting Na₂MnPO₄F was precipitated inacetonitrile including 0.6 M of LiBr dissolved therein, and reactedtogether with the flow of argon gas at a temperature of 80° C.

The test sample, in which ion exchange was completed, was washed withanhydrous ethanol to remove the remaining NaBr, and subsequently dried.Then, the resulting test sample was uniformly mixed with Super-P in aratio of 75:25 by ball-milling and then, prepared as a cathode materialcomposite.

Example 2

Sodium carbonate (Na₂CO₃), manganese oxalate.hydrate (MnC₂O₄.2H₂O),sodium fluoride (NaF), sodium hydrogen carbonate (NaHCO₃), and ammoniumphosphate (NH₄H₂PO₄) were introduced in predetermined amounts withrespect to the total amount of 10 g, and ball milled for 6 hours touniformly mix the materials.

The resulting mixture was subjected to pretreatment at 300° C. for 2hours under an air atmosphere, and fired at 500° C. for 6 hours under anargon gas atmosphere. Then, the resultant Na₂MnPO₄F was precipitated inacetonitrile including 1.0 M of LiBr dissolved therein, and reactedtogether with the flow of argon gas at a of 80° C.

The test sample, in which ion exchange was completed, was washed withanhydrous ethanol to remove the remaining NaBr, and subsequently dried.Then, the resulting test sample was uniformly mixed with Super-P in aratio of 75:25 by ball-milling, and then prepared as a cathode materialcomposite.

Example 3

Sodium carbonate (Na₂CO₃), manganese oxalate.hydrate (MnC₂O₄.2H₂O),sodium fluoride (NaF), sodium hydrogen carbonate (NaHCO₃), and ammoniumphosphate (NH₄H₂PO₄) were introduced in predetermined amounts withrespect to the total amount of 10 g, and ball milled for 6 hours touniformly mix the materials.

The resultant mixture was subjected to pretreatment at 300° C. for 2hours under an air atmosphere, and fired at 500° C. for 6 hours under anargon gas atmosphere. Then, the resulting Na₂MnPO₄F was precipitated inacetonitrile including 2.5 M of LiBr dissolved therein, and reactedtogether with the flow of argon gas at a temperature of 80° C.

The test sample, in which ion exchange was completed, was washed withanhydrous ethanol to remove remaining NaBr, and subsequently dried.Then, the resulting test sample was uniformly mixed with Super-P in aratio of 75:25 by ball-milling, and then prepared as a cathode materialcomposite.

Comparative Example 1

Na₂MnPO₄F obtained under the same condition as described in Example 1,without an ion exchange step, was uniformly mixed with Super-P in aratio of 75:25 by ball-milling, and then prepared as a cathode materialcomposite.

Comparative Examples 2 to 4

Sodium carbonate (Na₂CO₃), manganese oxalate.hydrate (MnC₂O₄.2H₂% sodiumfluoride (NaF), sodium hydrogen carbonate (NaHCO₃), and ammoniumphosphate (NH₄H₂PO₄) were introduced in predetermined amounts withrespect to the total amount of 10 g, and ball milled for 6 hours touniformly mix the materials.

The resulting mixture was subjected to pretreatment at 300° C. for 2hours under an air atmosphere, and fired at 500° C. for 10 hours underan argon gas atmosphere. Then, the resultant Na₂MnPO₄F was precipitatedin acetonitrile including 0.6 M (Comparative Example 2), 1.0 M(Comparative Example 3), or 2.5 M (Comparative Example 4) of LiBrdissolved therein, and reacted together with the flow of argon gas at atemperature of 80° C. The test sample, in which ion exchange wascompleted, was washed with anhydrous ethanol to remove the remainingNaBr, and subsequently dried. Then, the resulting test sample wasuniformly mixed with Super-P in a ratio of 75:25 by ball-milling, andthen prepared as a cathode material composite.

Comparative Examples 5 to 7

Sodium carbonate (Na₂CO₃), manganese oxalate.hydrate (MnC₂O₄.2H₂O),sodium fluoride (NaF), sodium hydrogen carbonate (NaHCO₃), and ammoniumphosphate (NH₄H₂PO₄) were introduced in predetermined amounts withrespect to the total amount of 10 g, and ball milled for 6 hours touniformly mix the materials.

The resulting mixture was subjected to pretreatment at 300° C. for 2hours under an air atmosphere, and fired at 550° C. for 3 hours under anargon gas atmosphere. Then, the resulting Na₂MnPO₄F was precipitated inacetonitrile including 0.6 M (Comparative Example 5), 1.0 M (ComparativeExample 6), 2.5 M (Comparative Example 7) of LiBr dissolved therein, andreacted together with the flow of argon gas at a temperature of 80° C.The test sample, in which ion exchange was completed, was washed withanhydrous ethanol to remove remaining NaBr, and subsequently dried.Then, the resulting test sample was uniformly mixed with Super-P in aratio of 75:25 by ball-milling, and then prepared as a cathode materialcomposite.

Comparative Examples 8 to 10

Sodium carbonate (Na₂CO₃), manganese oxalate.hydrate (MnC₂O₄.2H₂O),sodium fluoride (NaF), sodium hydrogen carbonate (NaHCO₃), and ammoniumphosphate (NH₄H₂PO₄) were introduced in predetermined amounts withrespect to the total amount of 10 g, and ball milled for 6 hours touniformly mix the materials.

The resulting mixture was subjected to pretreatment at 300° C. for 2hours under an air atmosphere, and fired at 550° C. for 6 hours under anargon gas atmosphere. Then, the resultant Na₂MnPO₄F was precipitated inacetonitrile including 0.6 M (Comparative Example 8), 1.0 M (ComparativeExample 9), 2.5 M (Comparative Example 10) of LiBr dissolved therein,and reacted together with the flow of argon gas, at a temperature of 80°C. The test sample, in which ion exchange was completed, was washed withanhydrous ethanol to remove remaining NaBr, and subsequently dried.Then, the resultant test sample was uniformly mixed with Super-P in aratio of 75:25 by ball-milling, and then prepared as a cathode materialcomposite.

Experimental Example 1 Test on Electrode Performance

The primary particle size of cathode materials prepared from Examples 1to 3, and Comparative Examples 1 to 10 was measured, and metalcomposition within the cathode materials was analyzed, by ICP emissionspectrochemical analysis. The results are noted in Table 1.

TABLE 1 Composition analysis LiBr result (molar ratio) molarity Primaryparticle size Li Na Mn PO₄ Exp. 1 0.6M 300 nm 0.3 1.7 1.0 1.0 Exp. 21.0M 300 nm 0.6 1.4 1.0 1.0 Exp. 3 2.5M 300 nm 1.9 0.1 1.0 1.0 Comp.Exp. 1 — 300 nm 0.0 2.0 1.0 1.0 Comp. Exp. 2 0.6M 400 nm 0.13 1.87 1.01.0 Comp. Exp. 3 1.0M 400 nm 0.3 1.70 1.0 1.0 Comp. Exp. 4 2.5M 400 nm0.65 1.35 1.0 1.0 Comp. Exp. 5 0.6M 500 nm 0.1 1.9 1.0 1.0 Comp. Exp. 61.0M 500 nm 0.2 1.8 1.0 1.0 Comp. Exp. 7 2.5M 500 nm 0.4 1.6 1.0 1.0Comp. Exp. 8 0.6M 700 nm 0.08 1.92 1.0 1.0 Comp. Exp. 9 1.0M 700 nm 0.171.83 1.0 1.0 Comp. Exp. 10 2.5M 700 nm 0.35 1.65 1.0 1.0

It was found that as the primary particle size decreased, the amount oflithium intercalated into Li_(x)Na_(2-x)MnPO₄F by ion exchange at thesame concentration of LiBr increased. In particular, when the primaryparticle size ranged from 700 nm to 400 nm, the amount of intercalatedlithium was slightly increased according to the decrease of the particlesize. However, when the particle size was 300 nm, the amount ofintercalated lithium was highly increased. This indicates that lithiumintercalation through a chemical method highly depends on the primaryparticle size. Accordingly, it has been found that in order tointercalate lithium into Na₂MnPO₄F through a chemical method such as ionexchange, it is important to control the particle size. Applicants havediscovered that the primary particle size required for effective lithiumintercalation by a chemical method is about 300 nm or less. Accordingly,the ball milling conditions and the heat treatment conditions of thestarting material are important. As described herein, the control of theparticle size was carried out by controlling the ball milling conditionsand the heat treatment conditions; however, one of ordinary skill in theart will understand that these conditions may vary according to thetypes of devices and procedures used for the aforementioned methods. Theimportant aspect is that chemical lithium intercalation is efficientonly when the primary particle size is controlled to a predeterminedsize or less. Accordingly, it is contemplated within the scope of theinvention that any methods that produce a primary particle size of about300 nm or less may be used. According to the invention, it is possibleto prepare Li_(x)Na_(2-x)MnPO₄F including both lithium and sodium.

By using powder of the cathode material composites from Examples 1 to 3and Comparative Examples 1 to 10, 95 wt % of cathode material compositewas mixed with 5 wt % of binding agent PVdF, and then a slurry wasprepared by using N-methylpyrrolidone (NMP) as a solvent.

The slurry was applied to aluminum (Al) foil with a thickness of 20 μm,and then dried and consolidated by press. The resulting product wasdried under a vacuum at 120° C. for 16 hours, to provide a circularelectrode with a diameter of 16 mm.

As a counter electrode, a lithium metal foil punched with a diameter of16 mm was used, and a polypropylene (PP) film was used as a separator.Also, as an electrolyte, a solution containing 1 M LiPF₆ in ethylenecarbonate (EC) and dimethoxy ethane (DME) mixed in a ratio of 1:1 (v/v)was used. The electrolyte was impregnated in the separator, and theseparator was positioned between the operating electrode and the counterelectrode. Then, the electrode performance of a battery was tested byusing a case (SUS) as an electrode test cell. The measurement resultsincluding discharge capacity are noted in Table 2 below.

TABLE 2 Discharge capacity at room Discharge temperature (mAhg⁻¹)voltage (V) Exp. 1 140 1.0 Exp. 2 178 1.0 Exp. 3 197 1.0 Comp. Exp. 1 551.0 Comp. Exp. 2 83 1.0 Comp. Exp. 3 112 1.0 Comp. Exp. 4 134 1.0 Comp.Exp. 5 72 1.0 Comp. Exp. 6 98 1.0 Comp. Exp. 7 129 1.0 Comp. Exp. 8 431.0 Comp. Exp. 9 54 1.0 Comp. Exp. 10 97 1.0

As shown in FIG. 2, when the surface of a test sample from Example 3 wasobserved by an electron microscope before and after ion exchangetreatment, it was observed that the surface of the test sample after ionexchange became rough due to deintercalation of sodium and intercalationof lithium.

The change in the charge/discharge characteristics resulting from ionexchange, was determined by comparing data from Example 1 andComparative Example 1 (see FIGS. 3 and 4). When ion exchange was notcarried out, Na₂MnPO₄F showed a discharge capacity of 55 mAhg⁻¹. On theother hand, when 0.3 of lithium was intercalated into Na₂MnPO₄F by ionexchange to produce Li_(0.3)Na_(1.7)Mn₂PO₄F, the discharge capacity was140 mAhg⁻¹, which was 2.5 times higher than that of Na₂MnPO₄F alone.Since only 0.3 lithium was intercalated, it was possible to achieve ahigher discharge capacity than Na₂MnPO₄F including only sodium. Thus, ithas been found that intercalation of lithium has a significantbeneficial effect by increasing the electrochemical capacity ofmanganese-based fluorophosphate. Additionally, as shown in FIG. 5,increasing the amount of intercalated lithium from 0.3 to 1.9 (Examples1 to 3), further increased the discharge capacity from 140 mAhg⁻¹ to 197mAhg⁻¹. From these results, it has been determined that intercalation oflithium into manganese-based fluorophosphate has a significantbeneficial effect by increasing electrochemical capacity. Without beingbound by theory, it is believed that this is because the intercalatedlithium establishes a pathway for lithium diffusion, which has apositive effect on lithium intercalation/deintercalation or sodiumintercalation/deintercalation during charging/discharging.

The invention has been described in detail with reference to exemplaryembodiments thereof. However, it will be appreciated by those skilled inthe art that changes may be made in these embodiments without departingfrom the principles and spirit of the invention, the scope of which isdefined in the appended claims and their equivalents.

What is claimed is:
 1. A cathode composition for a lithium secondarybattery, comprising: a compound of formula Li_(x)Na_(2-x)MnPO₄F, whereinthe compound of formula Li_(x)Na_(2-x)MnPO₄F consists of Li and Na, and0<x<2; and wherein the compound of formula Li_(x)Na_(2-x)MnPO₄F has aprimary particle size of about 300 nm or less so as to increase lithiumintercalating efficiency.
 2. The cathode composition of claim 1, whereinthe compound of formula Li_(x)Na_(2-x)MnPO₄F is coated with carbon. 3.The cathode composition of claim 2, wherein the coated compound offormula Li_(x)Na_(2-x)MnPO₄F has a potential plateau discharge of about3.7 V to about 4.0V.
 4. A method of producing a cathode composition fora lithium secondary battery, comprising: (i) mixing sodium (Na) oxide,or a precursor thereof, manganese (Mn) oxide, or a precursor thereof,phosphate (P), or a precursor thereof, and fluoride (F), or a precursorthereof by ball milling to produce a mixture; (ii) pretreating themixture of step (i) under an air atmosphere at about 300° C. for about 2hours; (iii) firing the pretreated mixture at about 500° C. to about550° C. to synthesize a compound of structure Na₂MnPO₄F; and (iv)intercalating a source of lithium into the compound of structureNa₂MnPO₄F produced from step (iii) by ion exchange to produce thecathode composition for the lithium secondary battery of structureLi_(x)Na_(2-x)MnPO₄F, wherein the cathode composition consistsessentially of Li and Na, and 0<x<2; and wherein the cathode compositionfor the lithium secondary battery of structure Li_(x)Na_(2-x)MnPO₄F hasa primary particle size of about 300 nm or less so as to increaselithium intercalating efficiency.
 5. The method of claim 4, wherein themixture of step (i) is mixed for about 6 hours by ball milling.
 6. Themethod of claim 4, wherein step (iv) further comprises intercalatinglithium ions into the compound of structure Na₂MnPO₄F produced from step(iii) by lithium intercalation/sodium deintercalation.
 7. The method ofclaim 4, wherein step (iv) further comprises the step of chemicallydeintercalating sodium from the compound of structure Na₂MnPO₄F producedfrom step (iii), and chemically intercalating lithium into the compoundof structure Na₂MnPO₄F.
 8. The method of claim 4, further comprising:(v) mixing the cathode composition of structure Li_(x)Na_(2-x)MnPO₄Fproduced by step (iv) with a carbon conductive material at a ratio ofabout 60:40 to about 90:10 by ball milling to produce a coating mixture;and (vi) coating a cathode surface with the coating mixture to improveelectric conductivity.
 9. The method of claim 8, wherein the carbonconductive material is selected from the group consisting of citricacid, sucrose, super-P, acetylene black, Ketchen Black, and carbon. 10.The method of claim 4, wherein the precursor of the sodium oxide isselected from the group consisting of sodium phosphate, sodiumcarbonate, sodium hydroxide, sodium acetate, sodium sulfate, sodiumsulfite, sodium fluoride, sodium chloride, sodium bromide, and anymixture thereof.
 11. The method of claim 4, wherein the precursor of themanganese oxide is selected from the group consisting of manganesemetal, manganese oxide, manganese oxalate, manganese acetate, manganesenitrate, and any mixture thereof.
 12. The method of claim 4, wherein theprecursor of the phosphate is selected from the group consisting ofammonium phosphate, sodium phosphate, potassium phosphate, and a mixturethereof.
 13. The method of claim 4, wherein the source of lithium isLiBr or LiI.
 14. The method of claim 11, wherein the ion exchange iscarried out with at least 0.5 M LiBr or at least 0.5 M LiI.
 15. A methodof producing a cathode surface with electrical conductivity, comprising:(i) mixing sodium (Na) oxide, or a precursor thereof, manganese (Mn)oxide, or a precursor thereof, phosphate (P), or a precursor thereof,and fluoride (F), or a precursor thereof by ball milling to produce amixture; (ii) pretreating the mixture of step (i) under an airatmosphere at about 300° C. for about 2 hours; (iii) firing thepretreated mixture at about 500° C. to about 550° C. to synthesize acompound of structure Na₂MnPO₄F; (iv) intercalating a source of lithiuminto the compound of structure Na₂MnPO₄F produced from step (iii) by ionexchange to produce the cathode composition for a lithium secondarybattery of structure Li_(x)Na_(2-x)MnPO₄F; (v) mixing the cathodecomposition of structure Li_(x)Na_(2-x)MnPO₄F produced by step (iv) witha carbon conductive material at a ratio of about 60:40 to about 90:10 byball milling to produce a coating mixture; and (vi) coating a cathodesurface with the coating mixture, thereby producing a cathode surfacewith electrical conductivity, wherein the cathode composition ofstructure Li_(x)Na_(2-x)MnPO₄F consists essentially of Li and Na, and0<x<2; and wherein the cathode composition of structureLi_(x)Na_(2-x)MnPO₄F has a primary particle size of about 300 nm or lessso as to increase lithium intercalating efficiency.